Build plugins dynamically for deb packages

Sphinx engine is built statically in deb packages, while it's dynamic in the binary tarball or in RPMs. There seems to be no reason to have it static, but there are reasons to have it dynamic.
It might also concern engines we add in future, we should be either careful on per-case basis, or do something generic so that plugins are built dynamically unless we specifically request otherwise.

_Also, blackhole, archive, federated and feedback are static in all packages, i don't know if there's a reason apart from the tradition. _